1. Field of Invention
The present invention generally relates to a locating system for determining the location of a transmitting station attached to an item or a person, and more particularly to a locating system for efficiently determining the positions of a number of transmitting stations indoors, such as in stores, warehouses, and offices, with a high precision without being subjected to much influence from environmental factors.
2. Description of the Related Art
Wireless tags (or transmitting stations) are used in various fields. For example, a tag system illustrated in FIG. 1 is actually working at many stores. The system shown in FIG. 1 comprises a gate 121 and wireless tags 123 attached to items or merchandise. If items 122a and 122b having tags 123a and 123b, respectively, pass through the gate 121 without paying at a cashier, the system generates alarms. This system is a so-called passive tag system using gates, in which passive tags are used as transmitting stations. The passive tag receives radio waves generated from the gate 121, modulates the radio waves, and returns signals to the gate 121. The gate 121 generates alarms when receiving the return signals transmitted from the tag 123. The passive tag system in combination with a gate is superior in maintenance because the tag (or the transmitting station) 123 does not require a power source. However, the communication range is limited to several tens of centimeters, and therefore it is unsuitable for a long-range tag system.
On the other hand, a so-called active tag system illustrated in FIG. 2 is known as a long-range tag system. In the active tag system, a power source is provided to each of the transmitting stations 127a-127f in order to extend the communication range. In general, the active tag system uses a frequency band assigned to a specific low power, and is capable of communicating in the range from several meters to several tens meters. However, this active radio tag system only has a function of determining presence or absence of the transmitting stations 127a-127f (tag 1 through tag 6) in the communication areas 125a and 125b of the receiving stations 124a and 124b, respectively. If the conventional active tag system (i.e., a combination of active transmitting stations 127 and receiving stations 124) is used to estimate the position of each transmitting station, the position estimating accuracy is beyond the communication range (i.e., exceeds the communication area size). In order to raise the positioning accuracy, the transmission power of the transmitting station must be lowered, or the sensitivity of the receiving station must be reduced, while increasing the number of receiving stations, to narrow the area covered by each receiving station.
To overcome the above-described problems of the active tag system, a locating system illustrated in FIG. 3, which is disclosed in JPA (Japanese Patent Laid-open Publication No.) 9-161177, is proposed. The system shown in FIG. 3 includes a transmitting station 131, three or more base stations 132a-132c, and a center station 133 communicating with the base stations 132. The transmitting station 131 transmits a signal containing the identification code of the transmitting station itself and current time (i.e., time of transmission) at a predetermined time interval using radio waves. Every time the base stations 132 receive the signal from the transmitting station 131, they each transmit the received signal, together with time of receipt and their identification codes, to the center station 133 by radio waves. The center station 133 calculates the distance between the transmitting station 131 and each of the base stations 132, based on the information received from the base stations 132, and estimates the position of the transmitting station 131. To be more precise, the center station 133 determines the signal propagation time from the time of transmission and time of receipt and calculates the distance between the transmitting station 131 and each of the base stations 132 by multiplying the propagation time with the propagation speed of radio waves. Then, the center station 133 estimates the position of the transmitting station 131 based on the positional relation with respect to the base stations 132.
The system disclosed in JPA9-161177 can estimate the position of the transmitting station by accurately measuring time. However, the signal transmission interval is generally set long at the transmitting station 131 in order to keep the life of the battery long. This causes a problem that precise position information can not be obtained when such position information is actually needed. In addition, at least three base stations 132 must be fixed in order to estimates the position of the transmitting station, and if the transmitting station moves out of the communication area of the fixed base station, the position of the transmitting station can not be estimated. Still another problem is that there is no information about the environment of the transmitting station.
FIG. 4 illustrates another known locating system, which is disclosed in JAP9-159746. The system shown in FIG. 4 includes a transmitting station 131 that transmits a radio signal, three or more base stations 132 that receive the radio signal from the transmitting station 131 and measure the intensity of the received signal, and a center station 133 that estimates the position of the transmitting station 131 based on the intensity of the received signal supplied from each base station 132. In this system, the transmitting station 131 generates and transmits radio signals during the positioning operation. Each of the base stations 132 supplies the measuring result of the signal intensity to the center station 133. The center station 133 calculates the distance between the transmitting station 131 and each of the base stations 132 from the intensity, and estimates the position of the transmitting station 131 based on the positional relation between the transmitting station 131 and each of the base stations 132.
A table listing the relations between intensities of the received signals and the corresponding distances is stored in the center station 133 in advance. The center station 133 determines the distance by applying the received intensity to the table. This system is capable of estimating the position of the transmitting station 131 by creating an accurate table indicating the relation between the intensity and the distance. However, in order to specify the position, at least three base stations must be fixed. If the transmitting station 131 moves away from the communication area of the base station, the position of the transmitting station 131 can not be estimated any longer.
Thus, the conventional “passive tag system” is unsuitable for a long-range radio tag system because its communication range is as short as several ten centimeters.
The conventional “active tag system” requires the number of receiving stations to be increased in order to improve the positioning accuracy.
The conventional locating system illustrated in FIG. 3, which estimates the distance based on the transmission time, needs to set the signal transmission interval long in order to keep the life of the battery of the transmitting station long. For this reason, it is difficult for this system to obtain accurate position information when it is actually required. In addition, at least three base stations have to be fixed to estimate the position, and if the transmitting station moves out of the communication area, the position of the transmitting station can not be estimated any longer. This system can not determine under what environment the transmitting station is operating.
The conventional locating system illustrated in FIG. 4, which determines the distance based on the intensity of the received signal, requires at least three base stations to be fixed. If the transmitting station is out of the communication area of the base station, the position can not be estimated any longer.
Although a positioning means making use of a GPS may be effective outdoors, it is unsuitable indoors because of reflected waves. Using an absolute time difference, as in a GPS, under the influence of reflected waves is ineffective because the error becomes too large. Even if estimating a position using amplitude information, the relation between the distance and the intensity of the received signal does not agree with the Friis' formula in many cases.
As is well known, Friis' formula is expressed by                     L        =                  20          ×                                    log              10                        ⁡                          (                                                4                  ⁢                  π                  ⁢                                                                           ⁢                  d                                λ                            )                                                          (        0        )            where L denotes the propagation loss, d denotes the distance, and λ denotes the wavelength.
The reason why Friis' formula does not work for indoors propagation is that the receiving station is located in hiding, or local fluctuation occurs in intensity of the received signal due to influence of reflected waves.